1. Field of the Invention
The invention relates to a fuel cell assembly and, more particularly, to a cell stack structure of a fuel cell assembly.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-Open Publication No. 2002-124291, or as shown in FIGS. 30 and 31, a FUEL CELL assembly, for example, a solid polymer electrolyte type fuel cell assembly 10 (FIG. 31), is formed by a stack of membrane-electrode assemblies (MEAs) and separators 18. The stacking direction is not limited to vertical directions but may be an arbitrary direction. Each membrane-electrode assembly is sandwiched by a pair of separators 18 to form a unit cell 19.
Each membrane-electrode assembly includes an electrolyte membrane 11 having an ion exchange membrane, an electrode (anode or fuel electrode) 14 having a catalytic layer 12 disposed on a surface of the electrolyte membrane 11, and an electrode (cathode or air electrode) 17 having a catalytic layer 15 disposed on another surface of the electrolyte membrane 11. Anode-side diffusion layers 13 and cathode-side diffusion layers 16 are provided between the membrane-electrode assemblies and the separators 18.
Each separator 18 has a fuel gas channel 27 for supplying a fuel gas (hydrogen) to the anode 14, and an oxidizing gas channel 28 for supplying an oxidizing gas (oxygen, or air in ordinary cases) to the cathode 17. Each separator further has a coolant channel 26 for passing a coolant (cooling water in ordinary cases) in a surface opposite from the channels 27, 28. Rubber gaskets 32 and adhesive seals 33 are provided in order to seal the channels 26, 27, 28.
On the anode side of each cell, a reaction occurs in which hydrogen is separated into hydrogen ions (protons) and electrons. The hydrogen ions migrate through the electrolyte membrane to the cathode side. On the cathode side, a reaction mentioned below occurs in which water is produced from oxygen, hydrogen ions and electrons (i.e., the electrons produced on the anode of the adjacent MEA come to the cathode through the separator, or the electrons produced on the anode of the cell disposed at an end in the cell stacking direction come to the cathode of the cell at the opposite end via an external circuit), whereby electricity is generated.Anode side: H2→2H++2e−Cathode side: 2H++2e−+(½)O2→H2O
In a conventional stacking method, modules are retained in the following manner. A spring 34 is disposed on an end of a cell stack in the cell stacking direction, and a swing portion 35 and an adjusting screw 36 are provided thereat. The modules of the stack 23 are retained with the spring force of the spring 34 providing a constant load in the cell stacking direction, and are retained in directions perpendicular to the cell stacking directions by the friction force of the spring force×the friction coefficient. In some cases, the modules are restrained from outside the cell stack through the use of an external restrainer member, in order to further reliably retain the modules in directions perpendicular to the cell stacking direction.
Further, document DE 100 49 801 A1 discloses a fuel cell assembly, wherein fuel cell modules are secured in the stack axis by an elastic holding plate that is coated with an insulating material. A spring applies a uniform pressure on the stack. The so build fuel cell assembly is enclosed within a container to prevent intrusion of impurities into the assembly.
Another fuel cell assembly is disclosed in document DE 195 45 11 A1. This assembly comprises a plurality of fuel cell modules surrounded by an elastic, having an insulating layer. Elastic members are provided to relieve mechanical movement of the modules due to thermal expansion.
Further, document U.S. Pat. No. 4,176,213 discloses a battery unit, containing one or more fuel-cell blocks, wherein the blocks are attached to a beam having an I-shaped cross-section, on both sides against the thin intermediate section thereof.
Document U.S. Pat. No. 5,824,199 A further discloses an electrochemical cell having an inflatable member, wherein a conductive inflatable member is provided between an electrode and a current for providing uniform contact pressure, and thus uniform electrical contact between the electrode and the current bus. The inflatable member comprises a pair of flexible plates, where one plate is thinner than the other.
Further, document JP 09092324 discloses a cell module which is formed of a cell layered product and a module forming member. Each module has a module frame clamping a plurality of single unit cells together, wherein the stacked modules are surrounded by an external enclosure. An insulation layer made from rubber or resin is provided between multi-cell modules and an inner wall of a frame as well as between an outer wall of the frame and the enclosure.
The conventional stacking method has the following problems.    1. When a cell stack having a total mass of M receives an impact of an acceleration a of several gravitational accelerations to about 20 G (G is the gravitational acceleration), a shearing force of Mα/2 occurs near an end portion of the cell stack. If the shearing force becomes greater than the spring force times the friction coefficient, sliding occurs between modules adjacent to the end portion of the cell stack so that the cell stack may disassemble.    2. If modules are retained from outside a cell stack by an external restrainer member, there is a risk of a stack end cell sticking onto the external restrainer member while moving in the cell stacking direction relatively to the external restrainer member as cell constituent members, such as MEAs, diffusion layers, etc., creep due to spring force. Thus, there is a risk of damage to cells. If the spring force is reduced in order to reduce the creep, attainment of a necessary inter-cell contact surface pressure is likely to become impossible.
A problem to be solved by the invention is that a cell stack disassembles as modules adjacent to an end portion of the stack slide upon an impact of acceleration in a direction perpendicular to the cell stacking direction (first problem).
Another problem to be solved by the invention is the disassembly of a cell stack upon an impact of acceleration in a direction perpendicular to the cell stacking direction combined with damages to a cell near an end portion of the stack caused in conjunction with the provision of an external restrainer member as the cell becomes stuck on the external restrainer member while cells move due to creep of cell constituent members (second problem).
It is an object of the invention to provide a fuel cell assembly capable of preventing disassembly of a cell stack by preventing modules adjacent to an end portion of the cell stack from sliding upon an impact of an acceleration in a direction perpendicular to the cell stacking direction (first object).
Another object of the invention is to provide a fuel cell assembly which is capable of preventing disassembly of a cell stack by preventing modules adjacent to an end portion of the cell stack from sliding upon an impact of an acceleration in a direction perpendicular to the cell stacking direction and which avoids the sticking of a cell adjacent to an end portion of the cell stack onto an external restrainer member if such a member is provided (second object).